Systems and methods for the removal of generally high-grade (high temperature) and medium-grade (medium temperature) waste heat is well known in the art. In particular, systems which employ the organic Rankine cycle (ORC) are used to convert high- and medium-grade waste heat energy into rotational mechanical energy that in turn can be coupled to an electric generator that may be connected to the commercial grid or used to provide independent power in locations where commercial power service is not available.
In the prior art ORC systems depicted in FIG. 1, heat energy may be directly supplied from the source to port 106 or its dedicated equivalent. The heated matter flows through heat exchanger 101 and exits at port 107 after transferring a portion of its latent heat energy to the separate but thermally coupled closed loop ORC system which typically employs an organic refrigerant as a working fluid. Under pressure from the system pump 105, the heated working fluid, predominantly in a gaseous state, is applied to the input port of expander 102, which may be a machine of various configurations including but not limited to a positive displacement twin screw expander, scroll expander, turbine, or the like. Here, the heated and pressurized working fluid is allowed to expand within the device, and such expansion produces rotational kinetic energy that is operatively coupled to drive electric generator 103 and produce electric power which then may be delivered to a local, isolated power grid or to the commercial power grid. The expanded working fluid at the output port of the expander, which may either be an essentially dry vapor or be a mixture of liquid and gaseous working fluid, is then delivered to condenser subsystem 104 where it is cooled until it has returned to its fully liquid state.
The condenser subsystem sometimes includes an array of air-cooled radiators or another system of equivalent performance through which the working fluid is circulated until it reaches the desired temperature and state, at which point it is applied to the input of system pump 105. System pump 105, typically a centrifugal pump, provides the motive force to pressurize the entire system and supply the liquid working fluid to heat exchanger 101, where it once again is heated by the energy supplied by the waste heat source and experiences a phase change to its gaseous state as the organic Rankine cycle repeats. The presence of working fluid throughout the closed loop system ensures that the process is continuous as long as sufficient heat energy is present at input port 106 to provide the requisite energy to heat the working fluid to the necessary temperature. See, for example, Langson U.S. 7,637,108 (“Power Compounder”) which is hereby incorporated by reference.
However, the ability of ORC systems to operate properly are often limited by the cooling resources available to remove residual, unconverted heat from their working fluid media during the Rankine cycle process of heating, expansion, cooling, and repressurization. For proper operation, a sufficient temperature differential (a minimum of 50° F., and preferably from 80° F.-100° F.) must exist between the waste heat input stream and the cooling resources available at the site. With a sufficiently high temperature waste heat input stream and/or consistently available cooling resources of sufficiently low temperature, ORC operation may be reliably achieved.
The practical limits of these conditions are being tested. The problem of low grade waste heat removal is very common in a variety of industries including automotive manufacturing, food processing, oil and natural gas processing, and computer data centers. Applications of this type generate vast quantities of waste heat in the range of 140°-190° F. in support of computer and data storage hardware, for example. At present, powered cooling systems are usually employed to remove this waste heat at significant additional expense to the operator. The trend is to locate these facilities in generally cooler (or cold) climates where the cooling requirements are reduced due to ambient conditions. Nonetheless, the cost of electric power to cool the facilities may still exceed to cost of electric power to operate the equipment being cooled.
In warmer climates, and even during the daytime hours of summer months in cooler climates, an insufficient temperature differential exists to permit power generation via the ORC process. With a higher temperature waste heat input and consistently favorable cooling resources, a conventional ORC waste heat power generation system may present a viable and cost-effective method of removing such heat. However, the generally low-grade heat produced by the applications discussed above would relegate an ORC system to part-time operation during frequent unfavorable cooling conditions and a supplemental cooling system would be required for periods when the ORC system was inoperable. As such, an ORC waste heat recovery system is not sufficient for use with applications that generate low grade waste heat in any but the coldest climates.
For the aforementioned reasons, there is a considerable need for a system capable of providing adequate management of low grade waste heat under the widest possible range of environmental conditions. The ideal system will preferably and advantageously convert as much energy as possible from the waste heat stream and convert it to other form(s) for beneficent use while consuming as little energy as possible for its own operation. Further, this ideal system will utilize as few components as possible to minimize cost, maximize reliability, and occupy as little physical space as possible.